1. Field of the Invention
The present invention relates to a method for making a grain-oriented silicon steel sheet having excellent magnetic properties that remain consistent between different production lots and within individual sheets.
2. Description of the Related Art
Grain-oriented silicon steel sheets are mainly used as iron core materials for transformers and other electric devices. Required magnetic properties of iron core materials include high magnetic induction at a magnetic field of 800 A/m (B.sub.8, in units T); low core loss, i.e., low alternating current core loss at 50 Hz in 1.7 T of the maximum magnetic induction (W.sub.17/50, in units W/kg); and the like.
Recent trends have required higher magnetic induction (B.sub.8 .gtoreq.1.92) in order to reduce core weight and noise. Furthermore, to improve fabrication efficiency and yield in large size transformers, homogeneous material characteristics are needed.
A grain-oriented silicon steel sheet is obtained by growing crystal grains of {110} &lt;001&gt; orientation, known as Goss orientation, by secondary recrystallization.
The following processes are involved in the production of a grain-oriented silicon steel sheet: heating and rolling at high temperature a silicon steel slab containing inhibitors required for secondary recrystallization, such as precipitates of MnS, MnSe, AlN and the like; cold-rolling the silicon steel sheet at low temperature at least once, or two or more times with intermediate annealing, to attain a final thickness; decarburization annealing the silicon steel sheet; applying an annealing separating agent such as MgO or the like to the steel sheet; and final annealing in the coil shape. Secondary recrystallization occurs during the final annealing process. An insulating coating comprising forsterite also forms during the final annealing process. Additional annealing after hot-rolling or during cold-rolling may be incorporated, and cold-rolling temperature may be raised as necessary.
Achieving further improvement in the magnetic properties requires a higher degree of secondary crystal grain growth in the Goss orientation. An effective means for producing such a result is to increase the rolling reduction to between 80-95% during final cold-rolling. However, when the rolling reduction during final cold-rolling reaches 80-95%, secondary recrystallization becomes very unstable, particularly in sheet steel less than 0.23 mm thick.
As a means for stabilizing secondary recrystallization when an increased rolling reduction is used during final cold-rolling, Japanese Patent Publication No. 62-50529 discloses a limited decarburization using AlN and MnS as principal inhibitors, such that carbon content is reduced by 0.0070 to 0.030 wt % after the hot-rolling process and before the cold-rolling process. However, B.sub.8 of the resulting products is only 1.92 T on average, thus the desired value of 1.92 T cannot be consistently obtained. Furthermore, the prior art does not disclose materials utilizing AlN and MnSe as principal inhibitors.
Because the coexistence of AlN and MnSe enables multi-modal precipitation, AlN and MnSe can finely disperse, thereby enhancing the inhibition effect. However, the presence of MnSe also renders insulating coating formation more difficult.
Japanese Patent Laid-Open No. 4-202713 discloses that controlling ambient temperature within a suitable range during the temperature elevation and soaking temperature in the decarburization annealing process improves coating properties and magnetic properties. The effects of oxides on the steel surface before the temperature increase, however, is not considered. Further, when this prior art technique is applied to materials containing AlN and MnSe as principal inhibitors magnetic properties over the entire product coil are inconsistent because secondary recrystallization at the middle portion of the coil is unstable.
As described above, no method for producing a coil-shaped, grain-oriented silicon steel sheet which possesses consistently excellent and stable magnetic properties has been found where AlN and MnSe are employed as inhibitors to promote high magnetic induction.